It is well known that ferromagnetic alloys lose their ferromagnetic property above a specific Curie-temperature.
It is also well known how to produce various ferromagnetic alloys of a different specific Curie-point. Typical application of such ferromagnetic alloys are found among other in electricity counters, relays, switches, electric motors, etc. where through a magnetic shunt eighter the effective magnetic flux of the magnetic system is held constant or a defined increase of the effective magnetic flux with a respective temperature is attained.
Furthermore there are thermo energy converter known such as: the Stirlingmotor, Solarmotor, Banks-Engine, etc. which mainly have the disadvantages, that they are complicated or climate dependant such as the solarmotor, or that an additional auxiliary energy source is needed.
To make a ferromagnetic disk turn in a directional magnetic field by changing its ferromagnetic property with the aid of say a blow lamp or a hot gas is also well known.